In a typical cellular radio system, a wireless telecommunication device communicates via one or more radio access networks (RAN) to one or more core networks. In a UMTS system such devices are typically referred to as User Equipment (UE) and in a GSM system such devices are typically referred to as Mobile Stations (MS). The terms can be considered as equivalent. In the description herein both terms may be used interchangeably, however, it will be noted that the term MS will be used predominantly since the present disclosure relates primarily to cell re-selection from a GSM EDGE Radio Access Network (GERAN) to a UMTS Terrestrial Radio Access Network (UTRAN) or an evolved UMTS Terrestrial Radio Access Network (e-UTRAN). It will be clear, however, that the present disclosure is not limited to this type of cell re-selection.
The Mobile Station (MS) comprises various types of equipment such as mobile telephones (also known as cellular or cell phones), laptops with wireless communication capability and personal digital assistants (PDAs) among others. These may be portable, hand held, pocket sized or installed in a vehicle for example and communicate voice or data signals or both with the radio access network (RAN). Of course the MS may not be mobile, but may be fixed in a location. In this context the term mobile may simply refer to the communication capabilities of the device.
In the following, reference will be made to Global System for Mobile Communications (GSM), Universal Mobile Telecommunications System (UMTS), Long Term Evolution (LTE) and to particular standards. However it should be understood that the present disclosure is not intended to be limited to any particular mobile telecommunications system or standard.
The Radio Access Network (RAN) covers a geographical area divided into a plurality of cells areas. Each cell area is served by at least one base station, which in UMTS may be referred to as a Node B or enhanced Node B in LTE. Each cell may be identified by a unique identifier which is broadcast in the cell. The base stations communicate at radio frequencies over a radio interface with the MSs which are camped on the cell (these may be some or all of the MSs which are within the range of the base station). Several base stations may be connected to a radio network controller (RNC) which controls various activities of the base stations. The RNCs are typically connected to a core network. Each cell implements a particular radio access technology (RAT) such as UMTS Terrestrial Radio Access (UTRA) among others. In a GERAN (Global system for mobile communications (GSM)/Enhanced Data rates for GSM Evolution (EDGE) radio access network), the radio access network may include one or more base stations (BTSs) and one or more Base station controllers (BSCs) which together implement the functionality of the base station subsystem (BSS) in respect of any particular cell.
When idle, a MS will evaluate the properties of detected telecommunications cells, other than the cell it is currently connected to or camped on (often known as the serving cell), in order to identify if the detected or candidate cells would be better suited for connection rather than the serving cell. The process of the MS autonomously changing the serving cell while in idle mode is known as cell re-selection (though re-selection may not be restricted to idle mode or purely autonomous cell change or both). The process by which a MS first camps on a cell following power-up is known as cell selection. The criteria for cell re-selection may include such things as received signal strength and signal quality. Parameters associated with these criteria may be broadcast or otherwise transmitted in the serving cell. One of the requirements for cell re-selection (and selection) may be that the candidate cell is suitable. The criteria for suitability may include criteria related to signal strength and/or signal quality (referring to the signal transmitted by the candidate cell base station, as received by the MS). Some of the criteria that are used in the suitability calculation are based on parameters which must be decoded by the MS after tuning to the frequency of the candidate cell. The decoding of the parameters from the candidate cell can be power intensive for the MS. Regular evaluation based on decoded suitability parameters in this way is particularly undesirable in mobile devices where battery life is limited.
The existing solution to this problem is to store and re-use the most recently decoded suitability criteria parameters. This solution is outlined in the 3GPP TS 45.008 specification V9.4.0, which is incorporated herein by reference and referred to herein as the ‘45.008 specification’. Section 6.6.5, section 6.6.6 and section 6.6.7 of the 45.008 specification describe the cell re-selection process, entitled “Algorithm for cell re-selection from GSM to UTRAN”, “Algorithm for inter-RAT cell re-selection based on priority information” and “Cell selection and re-selection to CSG cells and hybrid cells”, respectively. The algorithm defined in Section 6.6.5 may be described as the ‘ranking algorithm’ and the algorithm defined in Section 6.6.6 may be described as the ‘priority-based algorithm’. These terms may be used throughout the description herein.
According to the existing solution, when calculating the suitability of a candidate cell, suitability parameters of a cell from which the suitability parameters were most recently decoded may be used. This solution, although it may reduce the battery usage or long term power requirements of the MS, carries with it inherent limitations. Specifically, for example, it is easy for an MS to discount the suitability of candidate cell and thus not re-select to the cell when it should. This may be because the parameters used to calculate its suitability are incorrect, for example because the suitability parameters of the candidate cell are not the same as those that are being applied. Alternatively, a MS may incorrectly consider a cell as a valid cell for re-selection and proceed to, wastefully, tune to the candidate cell's frequency and decode parameters from that cell. This may be a particular problem if a wide range of threshold parameters are applicable to the cells that the MS could potentially reselect to. When one or more of these threshold are parameters stored, they may be used to incorrectly evaluate another cell in the network for which a different value is applicable.
In an example scenario, an operator may wish that devices in idle mode camp on cells of one frequency, and devices in connected mode, i.e. with active ongoing data or voice calls, operate in cells of a different frequency. The operator may attempt to achieve this by discouraging idle mode re-selection to a particular frequency by means of the suitability criteria, i.e. the suitability criteria are set such that an MS will be very unlikely to meet those criteria. In this scenario, the MS will consistently evaluate the cell for suitability using a considerable amount of power in the process. Additionally, if the MS has stored these incorrect parameters for re-use in the suitability calculation for other cells, the MS may not camp on a cell on which it should.
If a serving cell supports priority-based re-selection it may optionally transmit (including transmitting in broadcast messages) to an MS camped on the cell, an indication of a parameter to be used in the evaluation of a cell as part of a priority-based re-selection algorithm. However, support for priority-based re-selection is optional for non-LTE capable devices. For LTE capable devices, priority-based re-selection must be used. Priority-based re-selection is not supported in any pre-Release 7 devices.
Additionally, there is the possibility that priority-based re-selection is not applicable in the serving cell, for example if the cell is a GERAN cell. It is quite possible that operators will not upgrade GERAN networks to support priority-based re-selection, even when LTE is deployed, for example where there is sparse overlapping LTE coverage. Therefore LTE cells and nearby UTRAN cells most likely will support priority-based re-selection; overlapping or nearby GSM cells may not. A device supporting priority-based re-selection is still required to perform re-selection according to the ranking algorithm, if its current serving cell does not support priority-based re-selection. As such, there may arise scenarios, such as the examples given above, where priority-based re-selection, for whatever reason, is not applicable.
In a scenario in which priority-based re-selection is not applicable and the MS has stored previously decoded parameters, an MS may remain camped on a GSM cell longer than is appropriate when a UMTS or LTE capable cell is available but the MS does not consider it to be suitable (incorrectly). In addition or alternatively, power intensive decoding and calculations may be required on the part of the MS when in idle mode to determine if the cell is suitable re-selection. If the stored threshold is too high, you discount the candidate cell too easily; if the stored threshold is too low, you perform decoding and evaluation when it should not have been needed.
The present disclosure addresses the problem of how to efficiently and effectively evaluate a candidate cell when performing cell ranking re-selection. The present disclosure minimises the need to tune to the frequency of a candidate cell and decode suitability parameters from candidate cells while minimising the risk of discounting a candidate cell incorrectly.
The same reference numerals used in different figures denote similar elements.